One of the leaders of the Shiite-led opposition movement in Bahrain was arrested Monday by policemen wearing masks. Nabeel Rajab was sentenced to three months in prison for posting messages on Twitter that were judged to be “insulting” to residents of the city of Muharraq, who are, for the majority, Sunni.

Opposition activists have posted a video showing Rajab’s arrest on YouTube. In the footage, Rajab is taken out of his home in the capital Manama by a group of masked men wearing yellow vests marked “police.” Children are heard shouting as the dissident is escorted to a white van. One of the masked men, who is recording the scene, turns around for a moment and points his camera in the direction of the person filming the video.

Another video, posted to the same YouTube account, shows one of the masked men in front of Rajab’s house. He asks, “Is Nabeel inside?”

According to the state’s prosecutor, quoted by Bahrain News Agency, “A group of people from Muharraq lodged a complaint against the defendant for slandering them with insulting expressions.” The tweet in question came as part of a series lashing out against the prime minister, who has ruled the country for 41 years.

Rajab, who is the head of the Bahrain Centre for Human Rights, has also been accused by the authorities of taking part in unauthorised protests.

About three-quarters of Bahrain’s population is Shiite. However, it is ruled by a Sunni monarchy. Since February of last year, Shiites who feel they are being discriminated against have taken to the streets on a regular basis, calling for democratic reforms. These protests have often descended into violence between protesters and police.